Development and Internal Validation of a Prediction Model for Ureteral Access Sheath Insertion Depth During Flexible Ureteroscopy Based on Patient Anthropometric and Anatomical Parameters

In the operating room, a surgeon performing a delicate procedure on a kidney stone faces a silent challenge: how deep to push a thin, flexible tube into the body. This tube, known as a ureteral access sheath, acts as a protective tunnel, allowing the surgeon to guide a camera and tools up the ureter—the narrow channel connecting the kidney to the bladder—without damaging the delicate tissues. If the tube is pushed too far, it can poke a hole in the kidney or tear the lining; if it is not pushed far enough, the surgeon must repeatedly pull it back and push it forward under X-ray guidance, exposing the patient to more radiation and prolonging the surgery. For decades, the decision of where to stop has relied almost entirely on the surgeon's gut feeling, years of experience, and a rough guess based on the patient's size. There has been no reliable way to know the exact stopping point before the surgery even begins.

A team of researchers at Shanghai Jinshan District Central Hospital has set out to replace that guesswork with a precise calculation. They asked a simple question: could they predict the perfect depth for this tube by measuring a few things the patient already has, such as their height and weight, combined with a specific measurement taken from a standard preoperative X-ray? By studying 124 patients who underwent this surgery, the team developed a mathematical model that uses these everyday measurements to forecast exactly how far the tube needs to go. Their work suggests that by looking at the distance between two specific bony landmarks in the body, doctors can now estimate the correct depth with a high degree of accuracy, potentially making the procedure safer and faster for everyone involved.

The researchers focused on a specific anatomical relationship that had not been widely used for this purpose before. They measured the distance from the bottom of the first lumbar vertebra in the lower back down to the top of the pubic bone in the pelvis. In the human body, the ureter runs roughly along this vertical path, descending from the kidney area near the spine down to the bladder. The team hypothesized that this bony distance, which can be easily seen on a standard X-ray or a quick scan taken before surgery, would serve as a reliable ruler for the length of the ureteral path. They combined this measurement with other readily available data: the patient's height, age, sex, body mass index, and which side of the body was being treated.

To test their idea, the team gathered data from 124 consecutive adults who had undergone the procedure between April 2025 and July 2026. During each surgery, the medical team carefully recorded the exact depth the tube was inserted, measured from the skin at the perineum to the point where the ureter meets the kidney. They then fed all these numbers into a computer system designed to find patterns. The system tested six different mathematical approaches to see which one could best link the patient's physical traits to the actual depth used in the operating room. The goal was to find a method that was not only accurate but also simple enough for a doctor to use without needing complex software.

The results showed that a specific type of statistical model, known as ridge regression, worked the best. This model successfully predicted the correct insertion depth for nearly 79 percent of the patients within a margin of error of just two centimeters. In the world of this surgery, being within two centimeters is considered clinically acceptable because it means the tube will almost certainly reach the correct spot without needing constant readjustment. The model identified three factors as the most powerful predictors: whether the patient was male, their height, and the specific distance between the lower back vertebra and the pubic bone. Interestingly, the study found that the distance between the vertebra and the pubic bone provided unique information that height and weight alone could not capture, adding a significant layer of precision to the prediction.

The researchers also tested whether their new method was better than just using the bone measurement alone or just using standard body measurements like height and weight. They found that combining all the factors together was far superior. A model using only the bone distance was much less accurate, and a model using only height and weight was also less precise. By bringing them together, the team created a tool that explained more of the variation in depth than any single factor could. The study confirmed that the model was stable and did not just memorize the data from these 124 patients but could likely apply to others, as it performed consistently well even when tested on different subsets of the group.

Despite these promising results, the authors are careful to note that this is an internal study, meaning it was tested only on the patients from their own hospital. The group of patients was also predominantly male, which reflects the typical demographics of kidney stone surgery at their facility but means the model might need further testing to ensure it works equally well for women. The researchers emphasize that their tool is not meant to replace the surgeon's judgment or the need for X-ray confirmation during the operation. Instead, it serves as a planning aid, giving the surgeon a much better starting point than a guess. By narrowing down the range of where the tube should go, the tool could reduce the number of times a surgeon needs to adjust the tube, thereby shortening the surgery and reducing radiation exposure for the patient.

The ultimate goal of this research is to turn a subjective art into a more objective science. By measuring the distance from the lower back to the pelvis on a routine X-ray and plugging a few simple numbers into a calculator, a surgeon can now have a data-driven estimate of the correct depth before making the first incision. The team has made this calculation available as an online tool, allowing any doctor to use it immediately. While the study is a significant step forward, the authors acknowledge that the tool needs to be tested in larger, more diverse groups of people across different hospitals before it can be adopted as a standard part of the procedure worldwide. For now, it stands as a clear demonstration that a simple measurement, when combined with modern analysis, can bring greater precision to a complex medical task.